Buggy code:
public class FilenameUtils { private static final char EXTENSION_SEPARATOR = '.' ; private static final char UNIX_SEPARATOR = '//' ; private static final char WINDOWS_SEPARATOR = '\\' ; private static final char SYSTEM_SEPARATOR = File . separatorChar ; private static final char OTHER_SEPARATOR ; public FilenameUtils ( ) { } static boolean isSystemWindows ( ) { } private static boolean isSeparator ( char ch ) { } public static String normalize ( String filename ) { } public static String normalizeNoEndSeparator ( String filename ) { } private static String doNormalize ( String filename , boolean keepSeparator ) { } public static String concat ( String basePath , String fullFilenameToAdd ) { } public static String separatorsToUnix ( String path ) { } public static String separatorsToWindows ( String path ) { } public static String separatorsToSystem ( String path ) { } public static int getPrefixLength ( String filename ) { } public static int indexOfLastSeparator ( String filename ) { } public static int indexOfExtension ( String filename ) { } public static String getPrefix ( String filename ) { } public static String getPath ( String filename ) { } public static String getPathNoEndSeparator ( String filename ) { } private static String doGetPath ( String filename , int separatorAdd ) { } public static String getFullPath ( String filename ) { } public static String getFullPathNoEndSeparator ( String filename ) { } private static String doGetFullPath ( String filename , boolean includeSeparator ) { } public static String getName ( String filename ) { } public static String getBaseName ( String filename ) { } public static String getExtension ( String filename ) { } public static String removeExtension ( String filename ) { } public static boolean equals ( String filename1 , String filename2 ) { } public static boolean equalsOnSystem ( String filename1 , String filename2 ) { } public static boolean equalsNormalized ( String filename1 , String filename2 ) { } public static boolean equalsNormalizedOnSystem ( String filename1 , String filename2 ) { } public static boolean equals ( String filename1 , String filename2 , boolean normalized , IOCase caseSensitivity ) { if ( ( filename1 == null ) || ( filename2 == null ) ) { return filename1 == filename2 ; } if ( normalized ) { filename1 = FilenameUtils . normalize ( filename1 ) ; filename2 = FilenameUtils . normalize ( filename2 ) ; if ( ( filename1 == null ) || ( filename2 == null ) ) { <START_BUG> throw new IllegalArgumentException ( "Error<seq2seq4repair_space>normalizing<seq2seq4repair_space>one<seq2seq4repair_space>or<seq2seq4repair_space>both<seq2seq4repair_space>of<seq2seq4repair_space>the<seq2seq4repair_space>file<seq2seq4repair_space>names" ) ; <END_BUG> } } if ( caseSensitivity == null ) { caseSensitivity = IOCase . SENSITIVE ; } return caseSensitivity . checkEquals ( filename1 , filename2 ) ; } public static boolean isExtension ( String filename , String extension ) { } public static boolean isExtension ( String filename , String [ ] extensions ) { } public static boolean isExtension ( String filename , Collection extensions ) { } public static boolean wildcardMatch ( String filename , String wildcardMatcher ) { } public static boolean wildcardMatchOnSystem ( String filename , String wildcardMatcher ) { } public static boolean wildcardMatch ( String filename , String wildcardMatcher , IOCase caseSensitivity ) { } static String [ ] splitOnTokens ( String text ) { } }

Fix:
throw new NullPointerException ( "Error<seq2seq4repair_space>normalizing<seq2seq4repair_space>one<seq2seq4repair_space>or<seq2seq4repair_space>both<seq2seq4repair_space>of<seq2seq4repair_space>the<seq2seq4repair_space>file<seq2seq4repair_space>names" ) ;